Support vendor partition in non-debuggable pVMs

Use the existence of
/proc/device-tree/avf/vendor_hashtree_descriptor_root_digest (rather
than kernel param androidboot.microdroid.mount_vendor=1) to know if the
vendor partition is requested.

Bug: 340506965
Test: TH
Change-Id: I0ac1c773e44454fd9c52559d833dc8eca211889c